A web search engine searches for information on the World Wide Web in response to queries submitted by users. Search results can be presented in a list ordered by relevance and can include web pages, images, videos, information and other types of content deemed to be responsive to queries. Some search engines also mine data available in databases or other types of data repositories. A web crawler is a tool that systematically visits web sites in order to index their contents. Search engines use indexes built by web crawlers in order to find documents that are relevant to queries. When a user enters a query into a search engine, the engine examines its index and provides a listing of best-matching documents, usually with a short snippet containing the document's title and a portion of the document's text.